Whether it's paranoia or past experience, protecting your bag from potential thieves isn't a bad idea. It's also a fun DIY project! The folks over at MAKE figured out how to create a simple alarm out of headphone cables and a few other inexpensive items. Here's how it works:

This project uses simple electronics to create an alarm circuit for a shoulder bag. The key components are two 1/8" headphone jacks with "normally closed" switches, which detect whether a plug is in the jack. Wiring two of these jacks in series with an alarm creates a circuit that activates when both plugs are removed. With this arrangement, you can use one jack to keep the alarm from sounding while you arm the system. The other jack accepts a plug that you can tether to a belt loop or other anchor. When the bag is pulled away from its anchor plug, the alarm sounds. You can of course use this same circuit for a handbag, backpack, or any other personal cargo carrier.

To put this project together you'll need male and female headphone cables, a "AA" battery holder for eight batteries, eight AA batteries, battery snap connectors, a little plywood and clear plastic, heat-shrink tubing, and jewelry wrapping wire. You'll also need a bag to add this DIY alarm to, of course. This project is a little in-depth but seems pretty straightforward. Looks like a good one for the weekend so you can put it to good use when you're headed back to work on Monday.